Toward reliable XPS analysis of tin halide perovskites

Tin is the most promising replacement for lead in metal-halide perovskite solar absorbers. A key obstacle in their development is the facile oxidation of Sn(II) to Sn(IV), hampering optoelectronic performance and long-term stability. Investigating and ameliorating this issue requires precise quantification of Sn(II)/Sn(IV) ratios, often relying on X-ray photoemission spectroscopy (XPS). Here, we report that widely adopted fitting protocols of the Sn 3d spectra from perovskite surfaces, including those used in highly cited studies, can produce erroneous conclusions about the Sn states present. We therefore develop a robust fitting model for Sn 3d spectra, validating it using correlation analysis on a 50-spectrum dataset. In doing so, we reveal that Sn halide perovskite surfaces can be unstable under XPS measurement conditions, significantly altering the Sn(II)/Sn(IV) ratio under standard measurement durations of ∼1 h. Finally, we provide suggestions for XPS analysis of tin halide perovskites to avoid unreliable conclusions on oxidation state.